The long-range goals of this proposal are a) to determine in vivo the effects of acutely and chronically-altered plasma Ca levels on secretion and metabolism of parathyroid hormone (PTH) in dogs, using a new technique we call the "calcium clamp"; and b) to quantitate the effects of physiologic and pathologic stresses on plasma ionic Ca concentrations with a unique custom device, the Biostator (R) Ca ions Monitor (Life Science Instruments, Miles Laboratories, Elkhart, Ind.), for continuous on-line ionic Ca measurement. Many questions remain about fundamental aspects of the interplay between plasma Ca and PTH secretion and metabolism, which impact on interpretation of immunoassayable PTH data in clinical and physiological research, and medical practice. For example, alteration of ambient Ca concentrations in perfused organs appears to change metabolism of PTH, but the effect in vivo is uncertain. Some data suggest episodic secretion of PTH. It is not clear whether the relationship between plasma Ca, and PTH secretion is linear (proportional) or sigmoidal (derivative). To attack these and other questions, we have devised a method for external control of plasma Ca concentrations. Bolus injections of EGTA or Ca produce hypo-, hyper, or eucalcemia in less than 1 min; then algorithm-directed infusions of these agents at varying rates "clamp" the Ca at the desired values (C.V. less than 2 percent). We use rapid (2-4 min) Ca determination (microfluorometric EGTA titration) for feedback on infusion rates. Thus, we can induce "square wave" alterations of plasma Ca to study time- and concentration dependent effects on PTH secretion (PTH RIA specific for intact hormone and Ca homeostasis. Finally, we will integrate the Biostator (R) Ca ions Monitor with out Ca clamp technique for studies of ionic Ca perturbations by such stimuli as Ca feeding and normal meal ingestion, respiratory alkalosis and metabolic acidosis. Beyond answers to fundamental questions about in vivo PTH secretion and metabolism, and Ca homeostasis, our work may lead the way toward a Ca-regulating device for diagnostic and therapeutic use in severe hyper- and hypocalcemic states.